"Even though you almost conqueored the Earth Kingdom and the Water tribe with your ruthless Fire nation army and being like Riddler putting people into prisons and trying to conqueor the entire Earth I am not going to kill you instead I will take your bending away Ozai".
The thing is that Aang never killed anyone in the first place, and all the avatars before him had led a depressing life for deciding that the only way to rid evil is to kill it. The idea of making someone live with their failures is a harsher consequence than death, and it's never shown that Ozai ever returns to power at all. Furthermore this sets up Korra's first season as someone else is abusing the ability to take the bending away, which still shows that actions can have consequences. The difference however is that the removal of bending can always be reversed as it is just a blocking of chi, and straight up murdering someone, well, isn't.